Photographic process

The conventional process of photography relies on the light-induced reduction of silver halides to metallic silver. The silver halides (AgCl, AgBr) form 

The colloidal metallic silver formed in this way is opaque (it would appear black in reflected or transmitted light). The halogen atoms combine with an organic substrate mixed with the silver halide crystals to produce bromides. 

Although the primary photochemical process of photography is quite simple [it is an electron transfer from X (X = halogen) to Ag+ within a microcrystal of AgX], the technology of photographic emulsions and of their treatments after irradiation is complex. 

The photographic emulsion is essentially a dispersion of microcrystals of AgBr in an organic substrate called gelatin . The emulsion therefore consists of microscopic grains which have a specific size distribution. This distribution is important in determining the sensitivity and the resolution of the emulsion. 

During irradiation of the emulsion (exposure) those silver halide grains which have absorbed some light have one or a few silver cations reduced to metallic silver. These form a latent image on the emulsion, so called because it cannot be seen by the human eye on account of the very low concentration of metallic silver atoms. At this stage the photochemical process itself is over, the next steps in the processing of the exposed emulsion being dark (thermal) chemical reactions. 

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Bright yellow needles m.p. 45 C, b.p. 2 4°C. Prepared together with 4-nitrophenol by careful nitration of phenol. Sodium sulphide reduces it to 2-aminophenol which is used in dyestuffs and photographic processes. 

Metal salts of A-4-thiazoline-2-thione are used in the rubber industry Zn salts (123, 152), Pb and Mg salts (54). Cd salts (151, 324), Cu salts (325), in photographic processes (146). and in analysis (328). Zn, Ni, Co and Cd salts are used as germicides (329). Despite their wide range of application, little is known about their physical and chemical properties. 

Able to form Ag salt of lower solubility than AgQ in H2O. Therefore applications in photographic processes Inhibition of histidine decarboxylase activity Antifoggant for color films Anthelmintic activity Quenching for oil composition caialj si for the industrial isomerization of cis a, (3 unsaturaied carboxylic acids rubber vul-cankzate improver... 

Tetrasodium hexakiscyanoferrate decahydrate [14434-22-1], Na4[Fe(CN)g] IOH2O, or yellow pmssiate of soda, forms yellow monoclinic crystals that are soluble in water but insoluble in alcohol. It is slightly efflorescent at room temperature, but the anhydrous material, tetrasodium hexakiscyanoferrate [13601 -19-9], Na4[Fe(CN)J, is obtained at 100°C. The decahydrate is produced from calcium cyanide, iron(II) sulfate, and sodium carbonate in a process similar to that for the production of K4[Fe(CN)g] 3H2O. It is used in the manufacture of trisodium hexakiscyanoferrate, black and blue dyes, as a metal surface coating, and in photographic processing. 

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

G. Haist, Modem Photographic Processing]oha Wdey Sons, Inc., New York, 1979. 

L. F. A. Mason, Photographic Processing Chemistry, Focal Press, London, 1966. 

J. Kosar, Eight Sensitive Systems Chemistry andJipplications of Nonsilver Halide Photographic Processes, John Wiley Sons, Inc., New York, 1966 E. Brinckman, G. Delzeime, A. Poot, andj. WiUems, Unconventional Imaging Processes, Eocal Press, London, 1978. 

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